Dynamoelectric machines, for example electric motors, find numerous uses as prime movers. One application is fuel delivery where electric motors are used to drive pumps for providing pressurized fuel to combustion engines. In some of these "electric" fuel pump applications, the pump assembly and the electric motor are disposed within the tank from which fuel is being pumped. This arrangement, in fact, is typical for fuel delivery pumps and fuel boost pumps found on jet aircraft.
When used in fuel delivery applications, care must be taken to ensure a failure of the motor does not cause an ignition of the fuel, principally fuel vapor, within the tank. While extreme care is exercised to prevent motor failures, some motor failures are inevitable. One failure mode is caused by the wearing out over time in service of the insulation materials separating the coils of the motor stator. Heat is generated within the motor during normal operation which over time can lead to the degradation of the insulation materials. Excessive heat, such as resulting from application of excessive amounts of current to the motor or from application of current to the motor with the rotor in a locked condition, accelerates the breakdown of the insulation materials. Poor wiring connections, such as between the coil windings and fuses and/or terminal leads may also lead to a motor failure. The high resistance of a poor wiring connection may create excess heat, which can degrade or burn away insulation materials or potentially melt the wire forming the connection. A common result of these failure modes is the formation of a short-circuit between one or more of the stator coils.
In multi-phase motors, for example three-phase motors, a failure resulting in a short-circuit between the coils of two separate phases may result in a voltage differential, line-to-line voltage, up to .sqroot.3 times the line voltage. In a failure resulting in a short-circuit between a coil and neutral, a voltage differential up to the line voltage may result. In either failure event, a great deal of energy is released from the windings often as an electrical arc or plasma discharge. Discharges having sufficient energy to perforate the metal walls of multiple housing members within which the pump assembly and electric motor are contained have been observed. Perforation of the walls of the motor housing where the motor is used in fuel delivery applications may have significant detrimental effects including the potential ignition of fuel vapor within the fuel tank.
A hazardous yet typical location for the failure of a fuel pump motor stator winding is the end turns region of the motor stator. If a sufficiently large "air gap" is provided between the outer diameter of the stator and more particularly an outer diameter of the end turns of the stator and the motor housing wall inner diameter, enough of the energy of the arc is dissipated within the air gap such that perforation of the housing wall is substantially avoided. Additionally, controlling the above-described failure modes to both limit the occurrence of failure and the energy associated with a failure reduces the likelihood that a failure resulting in the perforation of the motor housing will occur.
Unconvinced that present electric fuel pump motors sufficiently protect against housing wall perforation in the event of fuel pump motor failure, the Federal Aviation Administration has issued directives requiring that certain jet aircraft maintain a minimum fuel quantity within the tanks at all times during operation. The minimum fuel quantity ensures that the electric fuel pump assembly remains submerged within liquid fuel while the aircraft is in service. Should an arc be discharged from the fuel pump due to a motor failure while the fuel pump is submerged in liquid fuel, there is insufficient oxygen available within the liquid fuel to result in ignition. However, maintaining a minimum quantity of fuel within the tanks significantly limits the range of aircraft. In fact, some commercial passenger long-range, non-stop flights have been discontinued as a result of these directives. These flights now require a refueling stop-over that adds to both the time and cost of the flight.